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Smoothness of space

  1. Sep 14, 2012 #1
    Last edited: Sep 14, 2012
  2. jcsd
  3. Sep 14, 2012 #2


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    Here FWIW is a technical article by Robert Nemiroff et al, the guy quoted in SciDaily about this.
    The reporting I've seen about non-dispersion results has tended to be somewhat naive. It is good these people continue to check to see that energy dependent speed of photons (dispersion) does not appear. But it is not predicted by a leading QG candidate (LQG) and so in a sense is not relevant to it.

    What this illustrates, as I see it, is that "smooth" can mean different things and you have to define operationally what you mean (e.g. it makes a difference whether or not you interrupted a process to find out "which slit" something went through.) That seeming paradox or non-intuitiveness can arise in measuring geometry just as with particles.

    Loop QG, for instance, does not postulate that space is a "foam" or that it is "made" of little grains, and it does not predict energy dependent speed of light. On the contrary, Lorentz invariance has been proven in Loop quantum geometry context. However the area and volume geometric MEASUREMENT operators have discrete spectra. This does not imply Lorentz violation! It's a fairly subtle point.
    So the propagation of light can be "smooth" over long uninterrupted distances, and yet if you measure an area defined by some events or material the possible outcomes of measurement are discrete (with smoothly varying probability) according to the LQG picture. It is somewhat analogous to measuring spin of particles---the outcomes of measuring spin orientation are discrete but it does not mean that space is "made" of some kind of grid with a fixed limited choice of directions.
    Although the article I linked was only recently published---8 June 2012 in PRL--- it was posted over a year ago on arxiv and was based on Fermi LAT observations made in May 2009 (as the SciDaily news item indicates). The source gammaray burst in this case (GRB 090510A) is numbered 0905....indicating the date.
    Last edited: Sep 14, 2012
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